The present disclosure relates to a method for determining the states of charge of battery cells or battery modules of a battery using a plurality of cell monitoring units, each of which monitors a plurality of the battery cells or at least one battery module. Furthermore, the disclosure relates to an associated battery management system for carrying out the method and also to a battery and a motor vehicle.
It is apparent that in future both in the case of static applications, such as wind power installations, in vehicles, such as in hybrid and electric vehicles, and in the consumer sector, such as in the case of laptops and mobile telephones, use will increasingly be made of new battery systems, on which very high demands in terms of the reliability, safety, performance and life thereof are placed.
Batteries with lithium ion technology are particularly suited to such tasks. They are distinguished by high energy density and low self-discharge, inter alia. By definition, lithium ion batteries comprise two or more lithium ion cells that are interconnected. Lithium ion cells can be interconnected by virtue of parallel or serial interconnection to form modules, and then to form batteries. Typically, a module comprises six or more cells.
DE 10 2009 046 567 A1 discloses a battery that is constructed from a plurality of battery modules, wherein the battery modules are monitored by means of a central battery management system.
In the present description, depending on the context, all of the battery cells arranged in battery modules with or without the associated battery management system can be referred to as a battery. In addition, the term cell monitoring unit can be abbreviated to CSC (cell supervision circuit), and state of charge to SOC.
The text below makes reference to FIG. 1 to describe an exemplary battery system 10 in accordance with the prior art that has a battery with battery cells 14 and also has a battery management system (BMS) 11. As indicated in FIG. 1 by a respective broken line, the battery system 10 has a battery with a large number of battery cells 14, and also has a large number of cell monitoring units 16. The battery cells 14 are grouped into battery modules, the precise split of the battery cells into the battery modules not being shown in FIG. 1. The battery management system 11 may be accommodated with battery cells 14 or battery modules in a shared housing (not shown). The battery modules may each have a separate housing. An arrangement of the battery cells 14 in battery modules can be used to achieve better scalability. In order to monitor the correct operation of the battery cells 14, the battery cells are monitored by a plurality of CSCs 16. A CSC 16 contains measuring electronics that monitor the voltage and further parameters. The information obtained by means of the CSC 16 is sent via a communication bus 35, for example a CAN bus, to a central controller 15 that evaluates the data from all battery cells 14 and, in the event of deviations from defined parameters, takes corrective action or if necessary opens the contactors 17, 18 and disconnects the battery system 10. The controller 15 is also connected to motor vehicle electronics (not shown) via the bus 28.
In order to monitor the correct operation of the battery cells 14, the battery management system 11 typically involves two respective modules being monitored by one CSC 16. In order to ensure sufficient functional safety for the battery system 10, the data from the CSCs 16 are evaluated and compared with one another both on the high voltage side 24 and on the low voltage side 22 of the controller 15 in two redundant microcontrollers 23, 25. In this case, the high-voltage-side microcontroller 25 uses the total voltage of the pack, that is to say of all battery modules, and also the total current that is measured by means of the shunt 26, for example. The low-voltage-side microcontroller 23 measures the voltage of the individual battery cells 14 and also the current that is ascertained using the Hall sensor 27, for example. Typically, the state of charge of the battery pack is also calculated on the low voltage side of the controller. To this end, the controller simultaneously checks current and voltage values for the battery cells that are relevant to the calculation of the SOC.
A disadvantage of such a battery system is that a large volume of data needs to pass via the communication lines to which the controller and the cell monitoring units are connected, which takes up increased bandwidth. This is particularly disadvantageous in the case of a motor vehicle in which a CAN bus or another bus system is used.